Information signals, such as data signals, media signals and especially compressed video and audio streams and more especially packetized audio and media streams propagate over various communication channels, such as but not limited to terrestrial, wireless, satellite, wireline and cable communication channels. Media streams usually include large amounts of information.
Digital transmission and compression techniques allow for transmitting media signals over communication channels in a compressed form. The Moving Pictures Experts Group (MPEG) specifications are standardized methods for compressing and transmitting media signals such as video and audio.
Communication channels that are connected to clients have a limited bandwidth, said limitation is known in the art as the “last mile problem”. Accordingly, only a certain amount of services can be provided to clients over said limited bandwidth channels. The amount of services is especially limited when some of them are bandwidth consuming.
It is noted that certain bandwidth limited networks are known in the art as broadband networks, but said title mostly emphasizes the much larger bandwidth these communication networks have (usually about 750 Mhz) in comparison to even more bandwidth limited communication networks and technologies, such as Plain Old Telephone Networks, dial-in modems and the like.
Clients are grouped in service groups. A service group includes a plurality of clients, such as set top boxes, cable modems and the like, that receive the same multiplexed downstream signal (‘downstream’ means from service provided to a client, while ‘upstream’ means information transmitted from the clients) as they share the same communication link/output port of a node, hub or even a Headend. A service group is also known in the art as a “forward carrier path”.
Typically, media packets are provided to Headends, primary hubs, secondary hubs and nodes and then are transmitted over communication networks to the clients. Some media packets can pass through more than one entity out of said Headend, hubs and nodes. In order to transmit the media packets over said networks they are modulated in various manners known in the art.
The modulation is implemented by modulators that may be located within said entities. A typical modulator is a Quadrature Amplitude Modulation (QAM) modulator, although other modulators, such as QPSK modulators, are known in the art. QPSK modulation is more robust than QAM modulation but is slower. It is noted that QPSK modulation may be used for modulating upstream information.
A QAM modulator usually receives input signals and outputs a modulated signal over a Radio Frequency carrier. Usually said modulator is also able to perform additional processing steps such as encryption, error correction coding, interleaving and the like. Most QAM modulators are able to alter the carrier frequency within a predefined frequency range. The modulators are arranged in arrays that are also known in the art as multi-modulators. Multi-modulators provide multiple modulated output signals. Modulated signals that are spaced apart in the frequency domain can be combined. Typically, each modulator outputs an output channel that has a bandwidth of 6 Mhz. The 6 Mhz channel can be used to convey a single analog television channel or ten MPEG compliant television channels. Typically, downstream channels are in the 50-750 Mhz band while upstream channels are in the 5-40 Mhz band.
Data is conveyed over various networks, including the Hybrid Cable Fiber (HFC) networks. A well known cable modem transmission standard is known as DOCSIS (Data Over Cable Service Interface Specification). Cable Modem Termination System (CMTS) are known in the art. They can be used for DOCSIS compliant systems.
A CMTS is usually installed in a cable headend or in a distribution hub and is connected to multiple cable modems via the HFC network. A conventional CMTS board transmits downstream information on a single downstream channel to multiple cable modems and receives information from multiple cable modems over one or more upstream channel. The upstream channel is usually termed out of band channel, as upstream transmission usually occupies a frequency range outside of the downstream frequency range.
The DOCSIS reference model includes a backbone network that is connected to a CMTS via a backbone transport adapter that in turn is also connected to a local server facility and a Telco Return Access Concentrator (TRAC). The TRAC is connected to a plain old telephone network. The CMTS is connected to data over cable security system DOCSS that enables data stream encryption. The CMTS is also connected to an operations support system and to a downstream combiner and upstream splitter that in turn are connected to a transmitter and to a receiver. The combiner can also be connected to video sources. These transmitter and receiver are connected to a hybrid fiber cable coax network that is connected to cable modems. The cable modems can be connected to client premises equipment. The CMTS is modeled as a network terminator that is connected to a modulator for downstream transmission and also connected to a de-modulator for upstream reception. DOCSIS is aimed to forward IP traffic transparently over a system that may be modeled by the mentioned above model.
There are various DOCSIS standards. The DOCSIS 1.0 is aimed to allow Internet access service to clients. DOCSIS 1.1 also supports telephony services. DOCSIS 2.0 adds the ability to use higher upstream data rates. The DOCSIS has a U.S version and a European version that is also referred to as EURO-DOCSIS. One of the differences between these version is the allocation of 6 Mhz wide channel in the U.S. and the allocation of 8 Mhz wide channels in Europe.
There are various models of communication protocols. A first well-known model is the OSI model that includes seven layers, starting from the physical layer (layer 1), data link layer (layer 2), network layer (layer 3), transport layer (layer 4), session layer (layer 5), presentation layer (layer 6) and application layer (layer 7).
A slightly different model describes the TCP/IP protocol suit that includes four layers such as the network interface layer, internet layer, the transport layer and the application layer. The network interface layer defines a standard interface for various low level layers. The internet layer provides routing and relaying functions for carrying packets of data from a source system to a destination system through an internet. This layer includes the Internet Protocol (IP), the Reverse Address Resolution Protocol, the Internet Control Message Protocol (ICMP) and the Address Resolution Protocol (ARP) that maps between logical network layer addresses (such as IP address) and hardware, data link Media Access Control (MAC) layer addresses. The transport layer provides an end-to-end data delivery service that is used to exchange messages over the internet, by application processes. This layer includes either the simple User Datagram Protocol (UDP) or the more complex but more reliable Transfer Control Protocol (TCP). The application layer includes many protocols that are aimed to provide a variety of services to network units. It includes, for example, the SNMP network management protocol, the TFTP file transfer protocol, the DHCP protocol that assigns IP address to devices, and the like.
CMTS and CM communicate using a multi-layered protocol stack that can be mapped to the TCP/IP protocol suit. When the CMs and CMTS operate as IP and LLC hosts the application layer includes SNMP, TFTP and DHCP protocols, the transport layer includes a UDP protocol, the internet layer includes IP, ARP and ICMP protocols, and the network interface layer include a variety of protocols and sub-layers starting from a Physical Media Dependent (PMD) sub-layer protocol, a Transmission convergence sub-layer (for downstream transmission only), a Media Access Control sub-layer, a link security sub-layer, and a Logical Link Control (LLC)/DIX/sub-layer. DIX is the Ethernet version 2.0 standard. In this context, DIX link layer framing refers to the “Type interpretation” of the Length/Type filed in ISO8802-3.
The transmission convergence layer defines a downstream data-conveying packet that has the same size and the same header format as the 188 byte long MPEG Transport packet (a.k.a. transport stream packet). Thus allowing both data and Video to be demodulated at the same manner and to facilitate common receiving hardware. A typical downstream MAC frame may includes MAC header and an optional variable length Ethernet type Packet Data Protocol Data Unit (PDU). This downstream MAC frame is preceded by a MPEG transmission convergence header. A typical upstream MAC frame is preceded by a PMD header.
The CMTS manages the upstream transmission by applying a media access control (MAC) scheme and allocating time slots and mini-time slots for upstream transmission. A single MAC sub-layer domain includes upstream and downstream channels for which a single Media Access Control Allocation and management protocol operates. One or more CMTS usually manage multiple MAC sub-layer domains. CMTS can support various quality of service classes by associating one of more service flow ID to each cable modem.
In order to effectively utilize the upstream bandwidth there is a need to synchronize the cable modems clocks as well as to assess the upstream delay from each cable modem to the CMTS. The CMTS generates and transmits SYNC messages that include a timestamp representative of when the SYNC message left the CMTS. The various upstream and downstream delays are determined during a ranging session that each cable modem undergoes before gaining access to the upstream channel. The cable modems receive the SYNC messages and synchronize their clock accordingly. A method for such synchronization by a cable modem is described at U.S. Pat. No. 6,698,022 of Wu titled “Time-stamp-based timing recovery for cable media access controller” which is incorporated herein by reference.
Another important MAC message is the Upstream Channel Descriptor (UCD) message that is used to number and to attribute mini-slots. A typical CMTS grant allows a certain cable modem to upstream transmit during a certain number of mini-slots, starting from a certain mini-slot.
The CMTS is also capable of determining the service group of each cable modem. This determination can be utilized by sending ping messages. A system, apparatus and computer readable medium that utilize ping messages in described at U.S. Pat. No. 6,594,305 of Roeck et al. titled “Media access layer ping protocol for diagnosing cable modem links”, which is incorporated herein by reference.
Various networks that include CMTS and can convey data according to DOCSIS are known in the art. The following patents and patent applications, all incorporated herein by reference, provide a brief overview of state of the art systems and methods: U.S. Pat. No. 6,711,135 of Dziekan et al., titled “HFC access network management system”; U.S. patent application 20030058887 of Dworkin et al., titled “Method and apparatus for ineterleaving DOCSIS data with an MPEG video stream”; U.S. patent application 20040019876 of Dravida et al., titled “Network architecture for intelligent network elements”; U.S. patent application 20040045035 of Cummings et al., titled “Distributed cable modem termination system (CMTS) architecture”.
DOCSIS compliant systems encrypt downstream data. One of the reasons for said encryption is the downstream transmission of data to multiple cable modems that share the same downstream channel. The security scheme is known as Baseline Privacy Interface (for DOCSIS version 1.0) or BPI+ (for DOCSIS version 1.1). This scheme includes using an encryption key of limited time duration to encrypt data being sent to cable modems. A cable modem is responsible to request a new encryption key once a current encryption key expires or nearly expires. BPI+ provides an improved scheme that uses a certificate based authentication such that a cable modem binds his MAC address to his RSA public key. The CMTS can verify the public key of the cable modem by verifying the authenticity of the certificate.
A brief overview of a BPI encryption scheme is illustrated in U.S patent application serial number 20030061623 of Denney et al., titled “Highly integrated media access control” which is incorporated herein by reference.